Thick film conductors serve as electrical interconnections between resistors, capacitors and integrated circuits. Thick film conductors are used in the microcircuit industry to "terminate" resistor or capacitor patterns, typically by firing the conductor pattern on a substrate and then printing the resistor or capacitor pattern over part of the conductor pattern and then firing them both.
Similarly to other thick film materials, thick film conductors are comprised of an active (conductive) metal and inorganic binder, both of which are in finely divided form and are dispersed in an organic vehicle. The conductive phase is ordinarily gold, palladium, silver, platinum or alloys thereof, the choice of which depends upon the particular performance characteristics which are sought, e.g., resistivity, solderability, solder leach resistance, migration resistance, bondability and the like.
The thick film techniques are contrasted with thin film techniques which involve deposition of particles by evaporation or sputtering. Thick film techniques are discussed in Handbook of Materials and Processes for Electronics, C. A. Harper, Editor, McGraw-Hill, N.Y., 1970, Chapter 12.
One of the most important factors in determining the adhesion performance of thick film conductors is the chemistry of the inorganic binder component. Three binder types are commonly used in thick film conductors: (1) glass; (2) oxide; and (3) mixtures of glass and oxide. In glass bonded systems, the glass typically migrates to the interface between the substrate and the conductive metal during firing and wets those surfaces. Fingers of glass extend from the substrate into the metal layer and sometimes even to the surface of the metal layer thus forming a mechanical bond.
A most common binder component in most solderable thick film conductor systems is bismuth oxide which facilities solderability and substrate adhesion. Bismuth oxide (Bi.sub.2 O.sub.3) functions in this manner by providing a "fluxing" action to remove unwanted oxides and glass. In addition, it improves the efficiency of other binder components such as glass and oxides by facilitating their flow and migration to the conductor-substrate interface.
However, it has been found that many inorganic materials such as Bi.sub.2 O.sub.3 nucleate and grow into a size far greater than the thickness of the printed and fired electrode film.
Such growths can protrude into the overprinted capacitor or resistor layers. Consequently, such crystalline functions can seriously interfere with the properties of the other thick film systems with which the conductor is used. For example, when a conductor of this type is used to terminate an overlying printed capacitor, the capacitor film is likely to exhibit degradation or even failure of its hermetic properties.
The degradation of the properties of resistor or capacitor layers terminated with conductor layers exhibiting such crystalline growth varies with the composition of the particular conductor. Nevertheless, it appears to be a function, inter alia, of the ionization-migration properties of the inorganic materials present in the underlying conductive composition.
The degradation and ultimate breakdown of dielectric properties is attributed to domain reorientation, ionization and ionic migration of the elements present in the dielectric materials, or impurities absorbed within the system, or ionization and migration of the ionizable species present in the electrode termination or all the above. Such ionization-migration of inorganic ions generally originates in the glass/frit component of the electrode composition. The ionization and migration of inorganic ions increases with the presence of easily ionizable inorganic oxides, i.e., when they are present as a separate phase, rather than as a component of the glass/frit.
A still further problem which is common with some thick film compositions is loss of silver during soldering, i.e., solder leaching, which drastically affects both the conductivity and solderability of the conductor. The problem is especially serious with silver-containing compositions. Furthermore, the problem is aggravated by the use of higher silver concentration, higher soldering temperatures and by longer times of exposure to hot solder.